Author: grant

Why there is not restart exception handling in PLT Scheme

YC asked here[1]:

Is it possible to handle exceptions like Common Lisp, specifically restarts?
Scenario: undefined variables – handling by defining the variable, and continue past the exception.

Matthew explained what is and is not possible in the following threads here, here, and here.
[1]: PLT Scheme Mailing list, “restart exception handling?”, Thu Jul 10 17:35:57 EDT 2008

Two sids of the same coin: coroutines and streams

Jos asked here:

It seems to me that in many cases the same problem can be solved (for an eager evaluater) both by coroutines and by streams. Both solutions very well show the structure of the principal algorithm as might have been written with a lazy evaluator in mind. Which approach would be preferred in terms of efficiency (time and memory). May be you would like to address other aspects as well. One reason that makes me tend to choose streams, is that the latter seem better suited to automatic code generation from purely lazy code.

To which Matthais replied here:

It is indeed a well-known fact that streams and coroutines are two sides of the same coin. Talcott’s 1986 dissertation is the earliest theoretical treatment that I know.

Which Jim clarified here:

I believe the reference is to Carolyn Talcott’s dissertation “The essence of Rum: A theory of intensional and extensional aspects of Lisp-type computation”. Stanford, 1986. Unfortunately this is not available online, as far as I know.

Hacking PLT to add a range syntax

Here is a post demonstrating how to add an infix range syntax to PLT Scheme.

[a .. b]  => (interval a b)
(... expr1 [expr2] ...) => (... (vector-ref expr1 expr2) ...)

Addendum: 04/20/09
Here is the final version of the code, with additional interesting syntax:

That includes:
[1 .. 4] short interval (1 to 3)
[1 … 4] long interval (1 to 4)
Curry with {}:
{+ _ 3} => (lambda (x) (+ x 3)
{+ _ (* 2 _)} => (lambda (x y) (+ x (* 2 y)))
This is convenient with map and filter:
(map {+ _ 3} [1 … 3]) => ‘(4 5 6)
Iota from SRFI 1:
[5] => (iota 5) => (0 1 2 3 4)
[5 3] => (iota 5 3) => (3 4 5 6 7)
[5 3 2] => (iota 5 3 2) => (3 5 7 9 11)
Lambdas:
{n -> (+ n 1)} is (lambda (n) (+ n 1)
And a very simple comprehension:
> [(+ n 3) : n is (< n 10) (even? n)] (3 5 7 9 11) > [(+ n 3) : n is (< n 10)] (3 4 5 6 7 8 9 10 11 12) > [(* n n) : n is (< n 10) (odd? n)] (1 9 25 49 81)

The PLT email list archive URLs have changed

The PLT Scheme email list archive URLs have changed. You can read all about it in this thread.
The result is that any links posted before this change was made are now broken.
Basically it is no one’s fault: not the PLT folks nor the sysadmins. The mailing list software reindexes messages in this breaking manner.
Nonetheless, now there are around 200 broken links on this site alone. I’m neither sure how to fix them automatically nor manually. Clearly the latter would be too time consuming.

Rudyard Kipling's If

Found here:

If you can keep your head when all about you
Are losing theirs and blaming it on you;
If you can trust yourself when all men doubt you,
But make allowance for their doubting too;
If you can wait and not be tired by waiting,
Or, being lied about, don’t deal in lies,
Or, being hated, don’t give way to hating,
And yet don’t look too good, nor talk too wise;
If you can dream – and not make dreams your master;
If you can think – and not make thoughts your aim;
If you can meet with triumph and disaster
And treat those two imposters just the same;
If you can bear to hear the truth you’ve spoken
Twisted by knaves to make a trap for fools,
Or watch the things you gave your life to broken,
And stoop and build ’em up with wornout tools;
If you can make one heap of all your winnings
And risk it on one turn of pitch-and-toss,
And lose, and start again at your beginnings
And never breath a word about your loss;
If you can force your heart and nerve and sinew
To serve your turn long after they are gone,
And so hold on when there is nothing in you
Except the Will which says to them: “Hold on”;
If you can talk with crowds and keep your virtue,
Or walk with kings – nor lose the common touch;
If neither foes nor loving friends can hurt you;
If all men count with you, but none too much;
If you can fill the unforgiving minute
With sixty seconds’ worth of distance run –
Yours is the Earth and everything that’s in it,
And – which is more – you’ll be a Man my son!

(via Bruce)